Routine water changes are the most basic, most necessary, and most overlooked acts of tank maintenance. Most aquarists know they should do water changes, but not everyone does or even knows how to do it the right way. How much and how often are highly debated topics among aquarists no matter what kind of tanks they keep.

Why should we do water changes?

Removing water from the aquarium and replacing it with new, “clean” water removes waste and organics that are dissolved in the water. It also helps to remove any chemical treatments or medications when the treatment is complete. Dissolved organics contribute to Nitrate and Phosphate build-up that aquarists try so hard to control. These compounds can affect the health of your livestock directly and can promote algae and cyanobacteria growth, making your tank unsightly. Changing the water also helps to replenish minerals and other trace elements. This can be especially important in tanks with corals and crustaceans (crabs and shrimp, both freshwater and saltwater) that use these minerals to form their skeleton or exoskeleton. Corals and other saltwater invertebrates can use up minerals fairly quickly in a closed environment, and replacing old, depleted water with fresh saltwater adds these minerals back into the tank.

What exactly is a “water change”?

Adding water to an aquarium to replace water lost due to evaporation is NOT a water change. This is simply “topping off” the aquarium. When water evaporates, all the waste, nutrients, chemicals, and other materials are left behind. This pollution must still be removed which is where the water change comes in; chemical filter media like carbon will remove some of these things, but not like a water change.

When should you do a water change, and how much water should you take out?

It may be some time before you have to perform a water change on a newly set-up aquarium. Once you add the first live or organic thing into the tank, be it fish or live rock or a bacterial additive, the Nitrogen Cycle will begin. It can take an aquarium anywhere from three weeks to 3 months or more to become “established”. During this time, you may see some algae growth and water chemistry changes. This is normal; hold off on your water changes until after this cycle is finished.

When it comes to established tanks, this is one of those eternal questions that may never have an definative answer, and almost every aquarist you ask will have a different opinion. I generally recommend changing 10-15 percent, once or twice a month once the cycle is complete, but there are a number of factors to consider.

In a tank with larger fish or fish that produce a lot of waste (like goldfish), water changes will probably need to be performed more often and with more of the water removed and replaced. Sensitive fish like Discus or reef tanks that need very low Nitrate levels may also need frequent water changes, but with only a small percentage of the water changed every time. Tanks with ample filtration may be able to go longer between water changes than tanks that have minimal filtration for their size and bio-load.

Other issues such as excess algae, high nitrate or phosphate levels, recent die-offs or illness in the tank, over-feeding and similar problems can affect your regular water change schedule.

Performing a Water Change

Before you start taking water out of your aquarium, prepare the water you’ll be putting back in. You want the new water to be as close as possible to the old water, especially in regards to temperature, salinity, and pH (only hopefully cleaner). I keep a clean 5-gallon bucket reserved specifically for water changes. If you are using tap water, you can add dechlorinator to remove the chlorine and chloramines. Test the temperature and use a spare heater to raise the temp of the replacement water if necessary. If you need to use any pH regulators or buffers in your aquarium, now is the time, but be sure to test the water before adding it to the tank. If you have a saltwater aquarium, mix the water ahead of time; a small spare pump can help you get the salt dissolved with little effort.

Next, take a look at your equipment. Do you need to turn your filter, pumps, protein skimmer or heater to prevent any damage if they run dry? Are all electrical outlets and lights safe from getting wet from drips or splashes?

You can use a cup or other container to scoop water out of the aquarium, but you can maintain the aquarium more efficiently if you use a gravel siphon. This is a rigid tube that connects to a length of flexible tubing that allows you to vacuum the substrate bed while removing water from the tank. Fish feces and leftover food in the substrate can cause infestations of organisms like snails, bristleworms, Aiptasia anemones and others creepy crawlies. Siphoning helps you clean the substrate and remove any leftover food or other detritus that may be decomposing and adding to the organics already dissolved in the water. The siphon allows heavier pieces like gravel fall back down to the bottom, while lighter debris (waste) is sucked out and into your bucket to be discarded. Just keep an eye on your fish, plants and other livestock to make sure nothing is getting sucked up by accident! If your substrate is especially fine (like sand) or coarse, you may need to experiment with siphon tube diameters to see which is right for you without being too weak or too strong.

Once you’ve taken out all the water you need to, the water can be disposed of down the drain. I usually hold a net over the drain to catch any bigger pieces of algae or gravel. Now the new water you’ve already prepared can be added to replenish the water level. Substrate, decorations or plants that might be pushed around when the water is poured in…try placing a small saucer, bowl or other container directly under where you are dumping the water to minimize the disturbance. Once the tank is refilled, any equipment that was turned off can be powered up again. For tanks with sensitive fish or livestock, you may wish to leave the lights off for an hour or so to let them relax after the activity of the water change. If your water change was part of a medication treatment regime, follow up with the instructions on the medication. If you are finished medicating, you can replace any chemical media like carbon now. Double-check your water chemistry in a few hours once the tank has been circulated through a couple times and you are finished! (…at least until next month…or next week…)

7 comments

Hello Eileen. Sorry, I don’t really have a question that pertains to this. I just wanted to make sure that this worked on my iPad. Could you please respond with something just so know I can use this in the future.

This is a great article i worked in several large chain pet stores around the country running the fish departments and this article covers everything i have explained to hundreds of customers. one thing i would add is that most modern filters have sponges in them to help filter the water and they also need to be cleaned every few months. it is important to never clean the filters in clean water or you will kill off any of the helpful bacteria that may be living in the sponge. just put the sponges in the bucket of dirty water you just removed from the tank and give them a good squeeze or two this will help clean the sponge so that it allows water to flow without killing the all that good bacterium that are needed in a healthy aquarium.

Under regular common steps to stablish a new aquarium, it must, specially at the beginning.
Accordingly with my research, there are achievable microbial communitys which in the right environment conditions will interact properly (with major living communitys) to promote long lasting desired water conditions, avoiding the need to do water changes at all. pH=7.6 NH4=0 NH3=0. One female livebearer fish spawn few minutes later, after been introduced to the little planted aquarium. Many authors had mentioned about the proper water conditionig by living aquatic plants, but still remainig good proper community interactions concepts. I think I have achieved some knowledge about the factors which helps to promote strong stable interacting complex community, which might be re-created.
Water changes might be necessary under common regular steps to stablish new aquariums mainly at the beginning but not when microbial community is pre-stablished, properly ordered, defined and supported achieving proper interacting climax community instead of an opportunistic one.

About Eileen Daub

Marine Biologist/Aquatic Husbandry Manager
I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there.
I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).

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